Vergo who first gave the impression of chambering and attacking with his left leg, changed leg and instead attacked with his right ankle (as you can see the inside of the foot is the one facing up and the stationary leg is aligned with the left hip, which means it can only be the left leg, and just see the streaks of action lines along the stationary leg that continue up the hip). This is just Vergo's way of signalling he is joking around with Sanji.
Next panel we see him swinging around his left leg for a reverse roundhouse kick. This means, and what is implied here, (but we don't get to see because Oda has a limited amount of panels so instead highlights Sanji's painful and fearful expression) he broke off the clash and left Sanji's leg hanging haplessly in the air, while he was fast enough to switch leg yet again using his left leg for a reverse roundhouse kick that Sanji barely dodges. Vergo is looking bored while doing this, as if he is just doing some light sparring with a complete noob.
Vergo toyed with Sanji in a playful manner, and the implication here is that Sanji has to be saved by plot, ie the emergency signal that revolves around Law breaching into the SAD room which Vergo has to listen in on. While Sanji is struggling for his life Vergo has to pay attention to the emergency message to divert his attention away from Sanji. A massive humiliation for Sanji.
Read this, absorb it, then think deeper about what it means :cheers: